1. Field of the Invention
The present invention relates to a method for producing impact resistant thermoplastic resins, and more in particular, for producing rubber modified impact resistant thermoplastic resins of high quality.
In essence the process comprises (a) emulsion graft polymerizing a minor amount of radical polymerizable mono-unsaturated monomers onto a major amount of diene rubber polymer particles in latex form, thus preventing any substantial aggregation of the rubber polymer particles in subsequent process steps, (b) mixing one or more radical polymerizable mono-unsaturated monomers with the grafted rubber polymer latex, if necessary with a coagulating agent such as salts or acids, extracting the grafted rubber polymer particles into the monomer phase without substantially changing or aggregating the rubber polymer particles in the latex, and separating and discarding the water phase to obtain a grafted rubber polymer particle dispersed monomer solution and then (c) bulk polymerizing the grafted rubber polymer particle dispersed monomer solution.
2. Description of the Prior Art
Generally speaking, rubber-modified impact resistant thermoplastic resins, so called ABS resins, are mainly produced by emulsion polymerization. The ABS resin obtained by conventional emulsion polymerization processes has excellent properties, but in the processes of recovering the polymer from the latex after polymerization, (i.e., in the processes of coagulation, dehydration, and drying) considerable deterioration of quality is encountered. Furthermore, the polymer recovery process is so complicated and troublesome that it is not satisfactory from an economic point of view.
In order to overcome the aforementioned drawbacks, various alternate methods have been devised in the past. One of the typical procedures is as follows.
According to one procedure of the prior art, the dry rubber, without gel, is dissolved into vinyl monomers, and the resulting viscous monomer solution (the rubber polymer is not dispersed but is dissolved homogeneously) is polymerized in bulk until the conversion reaches about 10 to 60%. During the pre-polymerization, according to the progress of polymerization, rubber polymer becomes precipitated and by agitation the precipitated rubber polymer is fixed as dispersed particles of a desired diameter in the monomer solution. (This phenomenon is called phase inversion). This phase inverted monomer solution is next subjected to polymerization such as suspension polymerization or bulk polymerization. This process is quite advantageous economically, because both suspension polymerization and bulk polymerization are quite simple in recovering the polymer from the polymerization system, compared with emulsion polymerization processes. However, the polymer quality is inferior to that obtained by emulsion polymerization processes, especially with respect to impact strength and surface gloss, both of which are essential properties for ABS resins.
For the same purpose, there has heretofore been proposed an improved process combining emulsion polymerization and suspension polymerization, in which water-soluble and oil-soluble initiators are simultaneously employed, in which event emulsion polymerization takes place in the primary polymerization stage, and the stability of the emulsion is destroyed in the course of the polymerization reaction by procedures such as partial coagulation attained by controlled addition of a coagulating agent in the presence of a suspending agent. As an example, reference may be made to Japanese patent publication No. 21073/1968, or the U.S. Pat. to deBell U.S. Pat. No. 3,370,105, granted Feb. 20, 1968. Nevertheless, these processes are still considered unsatisfactory in the following respects: (1) the polymer quality is still deteriorated by contamination from the emulsifier, suspending agent, initiator residue, or coagulating agent used, (2) production cost is still high compared with bulk polymerization, (3) the surface of the end product is inferior compared with that of conventional emulsion polymerized polymers or that of polymers obtained according to this invention.
On the other hand, it has been recognized through past studies on impact resistant resins that the essential properties of an ABS resin, such as its impact strength and surface gloss, have very strong relation to the condition of the rubber polymer particles dispersed in the resin and the state of grafting onto the rubber polymer particles, and moreover, the most desirable state of rubber particles and the state of grafting can be obtained when the rubber is provided in the form of latex.
We have discovered that it is possible to recover rubber polymer particles from latex in such a state that the particle size of the rubber in the latex can be substantially retained, and that it is possible simultaneously to disperse the rubber polymer particles in a mono-unsaturated monomer such as styrene and acrylonitrile to obtain a rubber polymer particle dispersed monomer solution, and that then it becomes possible to produce an impact resistant thermoplastic resin that has an excellent surface gloss directly by bulk polymerization; in this manner it is possible to overcome all the drawbacks of the conventional methods for producing impact resistant thermoplastic resins.
Relating to the methods of recovering polymer particles from latex retaining its particle size substantially and dispersing the polymer particle into monomers, and polymerizing the monomer solution to obtain a polymer particle dispersed resin, a procedure has been proposed rather recently in the United States patent to Sturt U.S. Pat. No. 3,663,655, granted May 16, 1972.
But even by this procedure it is impossible to prepare a particle dispersed monomer solution of a rubber polymer having a glass transition temperature which is below 0.degree. C, which solution enables one to produce rubber modified impact resistant thermoplastic resins, because the polymer latex, the polymer of which has a transition temperature of less than 20.degree. C, cannot be subjected to the extraction step in this procedure. The soft, rubbery polymer latex, having a glass transition temperature below 20.degree. C, cannot be used because the polymer particles in the latex are aggregated or agglomerated when mixed with monomers and passed into the monomer phase. So, in the aforementioned Sturt patent, a hard resinous polymer latex is chosen, such as PVC, polystyrene, styrene-acrylonitrile copolymer or acrylonitrile-polybutadiene-styrene copolymer. When the process according to Sturt is carried out by using an acrylonitrile-polybutadiene-styrene copolymer latex (ABS resin latex), a small amount of rubber polymer may exist in the resulting resin. However, the rubber content in the resulting polymer is too small to produce a rubber modified impact resistant thermoplastic resin.
Generally speaking, in ABS resin the rubber polymer exists only in a minor amount, say, 10 - 25% based upon the weight of the total polymer. On the other hand, the latex polymer/monomer ratio in the resulting polymer dispersed monomer solution is restricted to less than about 25% in order to conduct the next polymerization step. Therefore, the resulting resin contains only a very small amount of the rubber component, at most 7%, and has very poor impact strength such as 5 kg.cm/cm notched Izod impact strength.
The purpose of this invention is to obtain a method for dispersing a rubber polymer particle of which the glass transition temperature is below 0.degree.C, retaining the substantial latex particle size, into a mono-unsaturated monomer so as to obtain a rubber polymer particle dispersed monomer solution of which the rubber polymer content is high enough (up to 35% by weight) to give a very high impact resistance and excellent surface gloss by polymerizing the rubber polymer dispersed monomer solution.
And, moreover, in the process of the present invention the same merits both in polymer quality and in economy can be realized because the polymer recovery process is quite simple in bulk polymerization compared with emulsion or suspension polymerization, and the state of the grafted rubber particles dispersed in the resulting ABS resin is also the same as that of the ABS produced by conventional emulsion polymerization processes.